Lithium secondary batteries have a high capacity and a high energy density, and are easy to be reduced in size and weight, and therefore are widely used as power supplies for small-sized electronic devices for mobile use, e.g., mobile phones, personal digital assistants (PDAs), laptop personal computers, camcorders, and portable game machines, for example. Although small-sized electronic devices for mobile use are required to attain multiple functions, they are yet expected to overcome the cumbersomeness of battery exchange and achieve an enhanced device design. Therefore, there are increasing needs for structures in which a lithium secondary battery(s) is internalized within the device (lithium battery internalization). Moreover, lithium secondary batteries are expected not only as promising power supplies for small-sized electronic devices, but also as promising power supplies for large-sized devices, e.g., hybrid cars, electric vehicles, and power tools.
For adaptability in such applications, a higher capacity, and an improved durability and reliability, e.g., cycle life, are desired of lithium secondary batteries.
In order to attain a further increase in the capacity of lithium secondary batteries, development of positive-electrode active materials is under way. As the positive-electrode active materials, lithium-containing complex oxides are known, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) having a layer structure, lithium manganese spinel (LiMn2O4) having a spinel structure, and so on.
Among others, lithium nickel oxides such as LiNiO2 have a high reversible capacity (180 to 200 mAh/g) in a voltage range that is used for LiCoO2, and is capable of occluding and releasing larger amounts of lithium. Therefore, by using LiNiO2, lithium secondary batteries can be further increased in capacity, while minimizing side reactions such as decomposition of the electrolyte solution.
However, as compared to LiCoO2, there is a problem associated with LiNiO2: that is, a low operating potential of lithium occlusion and release. A low operating potential makes it difficult for the energy density of a lithium secondary battery to be further increased. Moreover, the poor stability of the crystal structure of LiNiO2 also presents a problem of a short charge/discharge cycle life. Furthermore, generally speaking, nickel-type lithium-containing complex oxides have an irreversible capacity which basically makes them unusable at room temperature. Therefore, there also exists a problem in that, when a battery is constructed by using a nickel-type lithium-containing complex oxide as the positive-electrode active material, an initial capacity of the battery is undermined.
Regarding the problem of the low operating potential, Patent Document 1 proposes a technique of using a mixture of a lithium nickel oxide with a lithium cobalt oxide, and carrying out a charging with a voltage which is higher than conventional.
Regarding the problem of a short cycle life, Patent Document 2 proposes using a positive-electrode active material in which a portion of the Ni in LiNiO2 is substituted by another element such as cobalt (Co) or aluminum (Al), in order to stabilize the crystal structure of the lithium nickel oxide. Non-Patent Document 1 proposes, in a lithium secondary battery whose positive-electrode active material is LiNiO2, carrying out charge/discharge while limiting the amounts of lithium occlusion and release with respect to LiNiO2. Specifically, it is stated that, when the lithium nickel oxide is expressed as Li1−yNiO2, the charge transfer resistance can be lowered by carrying out charge/discharge in a range such that y values are 0.15<y<0.75.
Patent Document 3 discloses a technique of prescribing an appropriate irreversible capacity (to be no less than 39 mAh/g and no more than 61 mAh/g) for the negative electrode so as to counteract the capacity losses associated with the irreversible capacity of a nickel-type lithium-containing complex oxide, thus minimizing the decrease in battery capacity.